1. Field of the Invention
This invention relates to the supplemental use of a switching power supply to draw power from a telephone line to assist a battery with powering the modem of a portable computer device. More particularly, this invention relates to a low noise constant input impedance switching power supply for drawing power from the input telephone line.
2. Background of Related Art
Many portable computer devices utilize modems for communications purposes. However, battery size and weight is an important consideration in the design and convenience of such devices. A balance must be reached between the size and weight of the overall device, dictated largely by the choice of battery, and an acceptable length of operation between charges.
Unfortunately, although operating an acceptable length of time when running typical application programs, portable computer devices utilize a large amount of power when communicating over a telephone line via a modem. The battery source which powers both the portable computer device and its modem is typically sized for general computing applications, and runs out of power quickly when actively communicating over a telephone line through a modem. Portable computer devices such as personal digital assistants (PDAs), hand held PCs (HPC), PCMCIA modems, and portable data terminals are designed to operate up to several hours on a single battery charge, but operate only fractions of an hour on a single battery charge when communicating via modem. Thus, although portable computer devices operate a sufficient length of time for quick data transfers over a modem powered completely by a battery, they require that external AC power be applied to allow for longer uses of the modem. It is therefore desirable for battery powered computer devices including a modem to draw power, in addition to the inherent battery, from a second power source.
The DC power inherent in a telephone line provides a convenient source of power, but there are often limitations and restrictions which limit the ability of a modem to derive power from the telephone line. For instance, present regulation in the United States is such that significant current may only be drawn from the telephone line when the telephone or modem is off hook or active. In order to hold the telephone line in an off hook condition, current in the approximate range of 20 mA to 150 mA must be drawn. The maximum amount of current available from the telephone line is also limited.
Modems which are designed to be powered entirely from the telephone line are known, but these designs suffer from an extremely constrained power budget. Moreover, modems are also subject to government constraints, e.g., FCC Part 68 requirements for telephones in the U.S., and limitations on effects and noise which may be placed back on the telephone line, placing further restrictions on the use of power from the telephone line.
Other conventional modems have inefficient power conversion apparatus making inefficient use of power from the telephone line.
In general, switching power supplies have previously been considered unsuitable for deriving power from the telephone lines because of the substantial amount of electrical noise inherent in such technology. A substantial amount of filtering is conventionally required to remove the noise generated back on the input lines, but this filtering would itself require a significant amount of power and thus degrade the net amount of power available from the telephone line. Switching power supplies are therefore conventionally avoided in the design of modems for computer devices not only because of the need for filtering but because of the additional stringent government standards as to the allowable maximum amount of disturbance of a telephone line due to a modem connection.